LA 3177 Beijing Guards(长城守卫)

#include <cstdio>
#include <algorithm>
using namespace std;
const int maxn = 100005;
int n,a[maxn],left[maxn],right[maxn];
bool ok(int p)
{
    int x = a[1],y = p-a[1];
    left[1] = a[1];right[1] = 0;
    for(int i = 2;i<=n;++i)
    {
        if(i%2==0)
        {
            left[i] = min(a[i],x-left[i-1]);
            right[i] = a[i]-left[i];
        }
        else
        {
            right[i] = min(a[i],y-right[i-1]);
            left[i] = a[i]-right[i];
        }
    }
    return left[n]==0;
}
int main()
{
//    freopen("in.txt","r",stdin);
    while(~scanf("%d",&n) && n)
    {
        for(int i = 1;i<=n;++i)scanf("%d",&a[i]);
        if(n==1){printf("%d\n",a[1]);continue;}
        int l = 0,r = 0;
        a[0] = a[n];
        for(int i = 0;i<n;++i)l = max(l,a[i]+a[i+1]);
        if(n%2==1)
        {
            for(int i = 1;i<=n;++i)r = max(r,3*a[i]);
            while(l<r)
            {
                int m = (l+r)/2;
                if(ok(m))r = m;
                else l = m+1;
            }
        }
        printf("%d\n",l);
    }
    return 0;
}